The present invention relates, in general, to an insulated gate power semiconductor device and, more particularly, to an insulated gate power metal oxide semiconductor transistor wherein the source is not shorted to the base.
Insulated gate semiconductor devices are devices employing a gate, or control electrode, insulatingly spaced from semiconductor material, for alternating conductivity of the semiconductor material beneath the gate. Typical insulated gate devices include power metal oxide semiconductor field effect transistors (MOSFETs), which are well known devices and insulated gate transistors (IGBTs). Both power MOSFETs and power IGBTs typically comprise a multitude of repeated, individual cells, with device current carrying capability increasing as cell size is made smaller. A base-to-source electrode short is typically employed in MOSFETs and IGBTs and, most commonly, comprises a portion of the source electrode electrically shorting together a P, or moderately doped, P-conductivity type, base region and a "N+", or highly doped, N-conductivity type, source region. This was done to prevent base-to-source PN junction between the P-base region and the N.sup.+ -source region from becoming forward biased which results in electron injection into the P-base region, across the base-to-source PN junction. Such electron injection may damage both MOSFETs and IGBTs. Until now, it was believed that a base-to-source electrode short was the best way to prevent this electron injection.
When the base/drift region diode becomes forward biased, large currents flow through the device. This is due to a comparatively low forward voltage drop of the forward biased junction, which is especially true when a P.sup.+ shorting region was used. Under forward bias, minority carriers were injected into the drift region increasing turn-off time of the forward biased diode and increasing power losses. Large reverse recovery currents required to compensate for the injected minority carriers could also turn on the parasitic bipolar transistor causing device failure.
One technique used to solve this problem was to use an external diode coupled in parallel to the base/drift region diode. In theory, the external diode would carry the large forward current, preventing transistor failure. In practice, however, a voltage drop across the base/drift region diode is so small that little current would flow through the external diode. What is needed is a power MOSFET transistor having a large base/drift region forward biased diode drop.